1. Field of the Invention
The present invention generally relates to charged particle projection systems and, more particularly, to projection lenses for electron beam and lithography tools.
2. Description of the Prior Art
Numerous industries, especially semiconductor integrated circuit manufacturing, rely on lithographic processes in which a pattern of material is deposited on or removed from a surface, such as etching a pattern into a substrate or a blanket layer of material. Lithographic processes are also used to make masks which may then be used in other lithographic processes. Generally, a layer of resist is applied to a surface and a selective exposure made of areas of the resist layer. The resist is then developed chemically to form a mask by removing either exposed or unexposed areas of the resist (depending on whether the resist is a positive or negative resist) and a material deposited or removed in a pattern corresponding to the mask such as by etching, implantation, chemical vapor deposition (CVD) or the like, possibly enhanced by the presence of a plasma.
To produce very fine features (e.g. fine pitch, small feature size and the like) very high resolution exposure is required. Resolution in optical exposure systems is limited by the wavelength of the radiation used to make the exposure as well as other physical effects presented by the exposure medium. Alternatively, beams of ions or electrons have been used to produce exposures at finer resolution than can be accomplished using even short wavelength (e.g. deep ultra-violet) light. Electron beam exposure is also convenient for complex patterns since an electron beam can be rapidly and accurately deflected by electrical and/or magnetic fields to serially expose selected areas of the resist such as in direct writing or step-and-repeat processes using a mask for shaping the electron beam.
However, some practical limitations on resolution are also characteristic of electron beams. Suitable resists for electron beam exposure require a significant electron flux (e.g. the number of electrons) for exposure and, as alluded to above, the exposure of respective areas is done serially. Therefore, throughput of an electron beam (hereinafter sometimes "e-beam") tool is limited by the beam current which can be developed. However, the charge carried by each electron or ion causes a repulsive force between the like-charged particles (generally referred to as Coulomb interactions) which increases with proximity between particles. Accordingly, high density of electron population in the electron beam causes aberrations in the nature of blurring or defocussing in the beam image because of the interactions between the electrons. Therefore, there is a trade-off between resolution/aberrations and maximum beam current and throughput.
At the present time, there are three principal approaches to increasing the useable beam current while containing electron interaction aberrations to a significant degree. Two of these approaches effectively rely on reduction of the average beam current density. The first approach involves the projection of relatively large sub-fields to maintain throughput at lower current density and, if the sub-field is sufficiently large, increased total beam current can be employed without severe detrimental effects of high current density. The second is to use a large numerical aperture which corresponds to a large beam semi-angle at the target (e.g. the average cross-section of the beam is large and sharply converged only shortly before the target through a large angle to the beam axis).
Both large sub-field sizes and large beam semi-angles put stringent demands on the aberration performance of the projection lens if high resolution is to be achieved since the geometric aberrations also increase with field size and beam semi-angle. Therefore there is an additional trade-off between geometric aberrations and electron interaction aberrations. These conflicting aberration requirements have led to adoption of a particular projection lens configuration known as a magnetic symmetric doublet. This type of lens configuration has particularly good geometric aberration performance which arises from the symmetry of the magnetic fields about the position of the common focal point or plane of symmetry of the two lenses of the doublet. If there is parallel illumination of the reticle (e.g. the illumination source is at infinity) the symmetry plane of the doublet coincides with the entrance pupil of the system. Depending on the type of reticle used in the projection system, there may be a physical aperture at the entrance pupil or, alternatively, the entrance pupil may be an image of a physical aperture which is in the illumination optics above the reticle.
More specifically, if the magnetic doublet has a magnification, m, and a z coordinate axis is defined along the axis of the cylindrically symmetric magnetic doublet lens with the origin at the plane of symmetry or entrance pupil, the required symmetry can be expressed as: EQU B(-z)=-(1/m)B(z*m) Equation 1
It can be shown that this symmetry relationship minimizes many of the aberrations of the projection lens, including, in particular, all the anisotropic aberrations and isotropic distortion. The relevant aberration integrals are provided in standard texts.
However, while the magnetic doublet provides good geometric aberration performance and thus allows some latitude in accommodating the trade-off between Coulomb interaction aberration (loss of resolution) and beam current (loss of throughput), throughput remains limited by resolution requirements, particularly as advances in integration density require smaller feature sizes and finer pitches.
A third approach to the trade-off which allows increase of resolution at a given throughput is to increase beam energy (e.g. a high accelerating potential for the beam). Geometric aberrations (with the exception of chromatic aberrations) are unaffected by beam energy while the trajectory displacement (TD) aberration due to Coulomb interactions and chromatic aberrations decreases with increased beam energy. As the approaches discussed above reduce electron proximity by increasing the beam cross-sectional area at a given current, increased electron energy decreases the time required for an electron to traverse the beam length and allows less time over which the Coulomb interactions can develop electron displacements and consequent aberrations. This can also be conceptualized as a decrease in electron density in the axial direction of the beam. Relativistic effects at high acceleration voltages further reduce TD aberrations. The relativistic contribution to the TD aberrations can be characterized by the parameter .GAMMA. where .GAMMA.=1+Vac/511 (Vac in kilovolts). Monte-Carlo calculations of the TD predict a (Vac).sup.-1.3 (.GAMMA.).sup.-1.2 dependence on beam accelerating voltage at constant beam current.
Unfortunately, resist sensitivity effectively declines with increased beam energy and increased beam current is needed for a given throughput, thus conflicting with the gain from increased beam energy. Therefore overall performance (considering both throughput and resolution) increases more slowly with increasing beam energy. Assuming that beam current must increase linearly with Vac to maintain constant throughput, Monte-Carlo calculations of TD predict a (Vac).sup.-0.6 (.GAMMA.).sup.-1.2 dependence on beam voltage at constant throughput.
Additionally, there are further practical difficulties with increasing beam energy and beam current since power dissipation is increased in lenses, deflectors, apertures and at the target and may not necessarily be accommodated. Moreover, at high beam accelerating voltage, it becomes increasingly difficult to provide reticle illumination with the necessary uniformity and beam semi-angle. Likelihood of beam damage of the reticle or mask is also increased and is believed to impose an upper limit on beam energy at the reticle of about 170-200 kV.
Conversely, reduced beam energy would allow economies throughout the e-beam system and, furthermore, a reduction in beam current since the resist sensitivity is effectively increased at lower beam energy. Unfortunately, with currently known projection lenses, axial chromatic aberration and TD aberration tend to increase as beam energy is reduced. Therefore currently known projection lenses which maintain constant beam energy throughout the trajectory are constrained to operate between a lower limit on beam energy imposed by TD and chromatic aberration and an upper limit on beam energy imposed by electron damage of the reticle and electron damage and power dissipation in the target.
In summary, currently known projection lenses are limited in their capacity for low geometric aberrations, low TD aberrations, low beam energy and high throughput. Further, currently known projection lenses cannot support further improvement in resolution without a severe reduction in throughput.